Wettability of a solid surface is important for various commercial applications. When a glass surface gets wet such as in the event of rain, for example, this may result in streaking of the glass surface. Upon drying, the glass surface becomes dirty due to segregation of dirt along the edges of the streaks or droplets as a result of the coffee-ring effect.
To circumvent the above, surface modification to obtain superhydrophilic surfaces having water droplet contact angle smaller than 10° have attracted great attention. The spread of water on superhydrophilic surface coatings may avoid streaking of dirt on surfaces to allow formation of cleaner surfaces. A superhydrophilic surface is useful for many applications involving self-cleaning and anti-fogging. Surfaces such as mirrors, windshields and building windows also require high transmittance in the visible range.
Superhydrophilic surfaces may be obtained using photocatalytically active materials such as titanium dioxide (TiO2), which become superhydrophilic upon illumination of UV light. These superhydrophilic films induced by photocatalytic activity, however, lose their superhydrophilicity a few minutes to hours after removal of the UV irradiation, or after storing in the dark. Furthermore, in case of titanium dioxide, the crystalline titanium dioxide coats may only be applied on surfaces at temperatures above 600° C. (e.g. Pilkington Activ). This limits application of the titanium dioxide coats to only materials that are able to withstand the high temperatures, hence temperature sensitive substrates such as plastics and organic substrates cannot be used. State of the art superhydrophilic films are also not photocatalytically active.
In view of the above, there remains a need for improved films or coatings and method of preparing the films or coatings that overcome or at least alleviate one or more of the above-mentioned problems.